This invention relates to a nuclear reactor operating method and a nuclear reactor, and more particularly to a nuclear reactor operating method and a nuclear reactor which are suitably applicable to boiling-water reactors for reducing consumption of nuclear fuel.
In conventional boiling-water reactors, a reactor core is installed inside a pressure vessel and is loaded with a large number of fuel assemblies, between which control rods operated by control rod drivers are inserted. The heat generation in the reactor is maintained by a self-sustaining fission chain reaction of fissile material (for example uranium-235) present in fuel rods. That is, in the reactor core, neutrons strike uranium atoms, resulting in the fission of the uranium atoms. The energy produced by the fission reaction is converted to thermal energy. Of the uranium, a uranium-235 is the fissionable material that is bombarded with neutrons to cause fission reactions. The uranium-235 accounts for only 0.7% of the naturally occurring uranium, with the remainder being unfissionable uranium-238. The uranium-235 is enriched to about several percent for use as a nuclear fuel.
In the conventional nuclear reactors, the chain reaction is maintained by operating the control rod and controlling the flow rate of coolant supplied to the core (hereinafter referred to as a core flow rate). The control rods absorb excess neutrons released by nuclear fission to control the chain reaction; and the core flow rate is adjusted to change the volume factor of water lacking part (a void fraction) by changing the amount of vapor bubbles in the core for controlling the chain reaction.
In boiling-water reactors using fuel assemblies (burnup: 0 GWd/T), as the core flow rate is increased, the void fraction generally decreases, promoting deceleration of neutrons, which is turn increases the neutron multiplication factor and therefore the reactivity. Now, the method of controlling the reactivity through absorption of excess neutrons and regulation of the void fraction will be described. We will explain the state of the reactor after a certain point in one fuel cycle (a fuel cycle is the period after the fuel assemblies have been loaded into the core and the reactor operation started until the reactor is stopped to replace spent fuel assemblies in the core), that is, after the reactor power has reached the rated output. At the initial period of the fuel cycle, the reactivity is potentially high, which requires the reactor operation to be performed in such a way as to raise the void fraction to reduce neutron deceleration, keeping reactivity at a desired level. As the burnup of fissile material proceeds and the reactivity lowers, the core flow rate is gradually increased to reduce the void fraction in the core, compensating for the reduction in reactivity. However, since the range in which the void fraction can be changed is small, a lower limit of void fraction is soon reached, making it impossible to continue compensating for the reduction in reactivity. To avoid this problem, a common practice is that the control rods are withdrawn to the exert corresponding to the amount of reactivity compensation obtained by the void fraction adjustment. Then, the core flow rate is again increased gradually to lower the void fraction and thereby compensate for the reduction of reactivity which accompanies the burnup of fissionable material.
For effective utilization of fuel, a reactor operation method is being considered which involves making the void fraction large at the initial stage of the fuel cycle to positively accumulate plutonium in the core and at the end of the fuel cycle burning the accumulated plutonium.
The void fraction may be changed by the foregoing described in the U.S. Pat. No. 4,716,007, or a method which adjusts the subcooling (the difference between the amount of energy per unit of mass of the cooling water at the saturated temperature and that of the cooling water entering the core).
The U.S. Pat. No. 4,716,007 describes the method of changing the void fraction in which slow-neutron absorbing water purge rods and neutron absorbing water purge rods made of stainless steel, which has a greater reactivity worth than the former, are provided and in which the amount of insertion into the core of the water purge rods is controlled to regulate the amount of cooling water in the core. The water purge rods constitute a means to change the void fraction in the core.
The advantages of progressively changing the number of hydrogen atoms in the core according to the burnup of nuclear fuel are explained below.
In the boiling-water reactors, a higher burnup is obtained when the reactor is operated with the void fraction set at high value (50%) at first and then lowered (to 30%) than when the void fraction is kept constant (at 30%) throughout the operation. This is because the larger the void fraction, the smaller the number of hydrogen atoms in the core and the less the neutrons will be decelerated. And the energy of neutrons remains higher, so that uranium-238 is converted into plutonium-239 at higher rates, retarding the reduction in the total amount of uranium-235 and plutonium-239. However, since the absolute value of the reactivity is small, the reactivity when the void fraction is high will reach the minimum level of reactivity to maintain criticality in a shorter period of time than when the void fraction is low. Thus when the minimum level of criticality is reached, the void fraction is lowered to cause neutron deceleration, which is turn increases reactivity. This enables the nuclear fuel to burn for a longer period than it does with the void fraction kept constant. The above reactor operating technique which makes use of void fraction changes for effective utilization of fuel is called a spectral shift operation.
The above spectral shift operation in which the water purge rods are manipulated requires the water purge rods and a device for driving these rods, making the reactor construction and its operation complicated. An example of the fuel assembly to which the spectrum shift operation is applicable is disclosed in the Japanese Patent Application Laid-Open No. 38589/1986. The fuel assembly has a heater in a water rod. However, since the heater is formed of a low-enriched fuel rod, the structure of the fuel assembly is complex and its manufacture is not easy.
In this respect, the spectrum shift operation that regulates the core flow rate can eliminate such problems. The spectrum shift operation, however, has the following problems The lower limit of the core flow rate is restricted by thermal limitations, and the upper limit is constrained by the performance of a circulation pump and a heat exchanger as well as by flow vibrations. Thus, with the boiling-water reactor operating at the rated power, the void fraction can only be changed in a narrow range with its center at a void fraction which corresponds to the core flow rate for the 100% output rating. For example, suppose the range in which the core flow rate can be varied is 80 to 120%. Then the range in which the void fraction is varied is about 9%. With such a small variation range for the void fraction, the spectrum shift operation will not be effective. This also applies to the reactor operation using the fuel assembly described in the Japanese Patent Application Laid-Open No. 38589/1986.